Delivery of electrical shocks has proven to be an effective and relatively safe means for defibrillation of the heart, or for treating various types of cardiac arrhythmias. Implantable defibrillators, cardioverters, and combined pacemaker/cardioverter/defibrillator devices have come into widespread use for treatment of various arrhythmias. As effective as these new devices and systems have been, there remain a number of areas where substantial development work is needed, in order to increase efficiency and reliability, and to deal with known adverse effects. Some of the known problem areas are those of tissue damage in the vicinity of the electrodes across which the shocks are delivered, and in the case of atrial defibrillation, the patient sensation of sharp pain which can be caused by over-stimulation of the phrenic nerve. Despite improvements in lead and electrode design, there remains an ongoing need to reduce the amount of energy delivered in a shock, both for purposes of preserving the battery energy in the implanted device and for the important purpose of reducing patient anxiety about receiving a shock.
One alternative approach to the area of defibrillation that has been considered but not given much development is using ultrasound for defibrillating. This involves delivering high energy ultrasound waves to the affected region, e.g., the atrium or ventricle. Unfortunately, this technique has thus far met with a low success rate, on the order of 30%, and has been seen to cause damage to the endothelium and to the cardiac muscle fibers. However, it is the basis of this invention that lower frequency mechanical agitation of the heart has a variety of potential beneficial effects which have not yet been successfully exploited, particularly in treating a fibrillating heart or a heart which is prone to fibrillation or other arrhythmias. Mechanical agitation of the heart, particularly at frequencies below about 50 kHz, can have a direct defibrillation effect, as well as other beneficial effects on the fibrillating heart. Mechanical agitation, or movement of the cardiac muscle with sufficient energy can change the conductive properties of the muscle, and thus change the loop gain and disrupt the circular conductive paths which had been responsible for the fibrillation or other arrhythmia. Likewise, mechanical agitation may affect action potential duration of myocardial cells, and thus aid in termination of fibrillation. Mechanical agitation directed to the cardiac muscle may facilitate muscle fiber relaxation, and help bring the fibers into diastole. This, in turn, can reduce the threshold for electrical defibrillation. Similarly, mechanical treatment may be used to improve cardiac perfusion. It is known that perfusion of the myocardium is limited to the diastolic phase, and that during fibrillation perfusion stops even though the aortic pressure remains high for a few seconds. Application of low frequency mechanical waves to the myocardium can have a relaxing effect which improves perfusion, as well as a direct massaging effect which aids perfusion.
In addition to direct mechanical treatment of the heart, mechanical vibration of the lungs may also be used to treat a fibrillation problem. Low frequency vibration is known to improve gas exchange in the lungs. Since oxygen supply and carbon dioxide are critical in a fibrillation condition, such mechanical vibration to provide enhanced gas exchange in the lungs is supportive of and beneficial to defibrillation. Further, depending upon the precise effects of the low frequency waves at the air-tissue interface in the lungs, such waves may have the property of stimulating the pulmonary stretch receptors. This will, via the cardio-pulmonary reflex, result in parasympathetic output (n.Vagus) in the heart and contribute to suppressing fibrillation. In view of these considerations, it is seen that there is substantial potential for therapeutic application of implantable low frequency mechanical wave devices for treating arrhythmias, particularly fibrillation and transient ischemic heart disease.